The present invention relates to a low frequency sound generator comprising a feeder unit and a quarter wave resonator tube, particularly an air-driven low frequency sound generator with a positive feedback system of the type described in U.S. Pat. No. 5,005,511. The feeder unit is connected to a compressed air system. A low frequency sound is generated by air pulses fed by the feeder unit into the resonator tube
The dimensions of the resonator tube are chosen in such a way that a standing sound wave may be generated in the resonator tube by the compressed air supplied via the feeder unit. Preferably, the resonator tube is a quarter wave resonator tube. When a standing sound wave has been generated in the resonator tube, this standing sound wave has its maximum sound pressure amplitude at the rear end where the feeder unit is positioned.
The rear end of the resonator tube is exposed to the maximum sound pressure, resulting in a force acting on the rear end of the resonator tube. That force varies between a positive maximum value and a negative minimum value. The frequency of that variation is equal to the frequency of the standing sound wave in the resonator tube. The maximum positive value of the force can be up to 8 000 N or even, at some cases, higher.
The variation of the force acting at the rear end of the resonator tube of the low frequency sound generator causes an acceleration of the low frequency sound generator which also varies in phase with the sound pressure and having the same frequency as the frequency of the sound pressure.
The acceleration results in a movement of the low frequency sound generator. The maximum movement velocity amplitude depends on the acoustic power output of the low frequency sound generator, the mass of the low frequency sound generator and the way of suspension of the low frequency sound generator. At big powerful low frequency sound generators that movement can have a velocity amplitude of up to 100 mm/s or even more.
Low frequency sound generators of this type are, for example, used for cleaning of big boilers and for enhancing combustion in big furnaces.
The low frequency sound generator is fitted to the outer wall of the boiler or furnace. A movement of the low frequency sound generator can cause vibrations of the boiler or furnace, structure-borne noise of the boiler or furnace and other equipment connected to the boiler or furnace and/or air borne noise at the surroundings of the low frequency sound generator and the boiler or furnace.
Vibrations caused by low frequency sound generators have been an ever increasing problem as the acoustic power output of the low frequency sound generators have continuously increased over the years. So far two different technical solutions have been used to overcome the problems.
One technical solution is to make a very stiff connection between the resonator tube of the low frequency sound generator and the wall, where the low frequency sound generator is fitted.
The other technical solution is to fit ballast weights to the resonator tube, hence increasing the mass of the low frequency sound generator and the total vibrating system.
At low frequency sound generators having high acoustic power output none of above mentioned technical solutions works well.
There is a need for a device that prevents low frequency sound generators to vibrate. Preferably, the new device should not substantially increase the weight of the low frequency sound generator. There is a need for a device that can be easily installed on existing low frequency sound generators.